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Abstract:

The invention provides an organic light-emitting diode which includes at
least two electroluminescent layers (ELR, ELB), both of which are
fluorescent or phosphorescent and emit at different wavelengths, as well
as a hole- and electron-conducting buffer layer (T) arranged between the
electroluminescent layers. The buffer layer is a bi-layer having an
electron-transport layer (T2) and a hole-transport layer (T1), each one
of the hole- and electron-transport layers being made of one or more
materials in which the HOMO level(s) are comprised between or equal to
the HOMO levels of the electroluminescent layers, and in which the LUMO
levels are between or equal to the LUMO levels of said electroluminescent
layers, with a tolerance of 0.3 eV.

Claims:

1. An organic light-emitting diode comprising at least two
electroluminescent layers (ELR, ELB), both of fluorescent type or of
phosphorescent type and emitting at two different wavelengths, as well as
an electron-and-hole conducting buffer layer (T) disposed between said
electroluminescent layers, characterized in that said buffer layer is a
bilayer comprising an electron-transport layer (T2) and a
hole-transport layer (T1), each of said electron-transport and
hole-transport layers consisting of one or more materials whose HOMO
level or levels lie between, or are equal to, the HOMO levels of said
electroluminescent layers, and whose LUMO levels lie between, or are
equal to, the LUMO levels of said electroluminescent layers, this being
with a tolerance of 0.3 eV.

2. The organic light-emitting diode as claimed in claim 1, in which said
electroluminescent layers comprise a matrix and an electroluminescent
dopant, and in which the two layers of said bilayer comprise a matrix of
the same composition as that of the closest electroluminescent layer.

3. The organic light-emitting diode as claimed in one of the preceding
claims, in which said electroluminescent layers comprise a matrix and an
electroluminescent dopant, and in which said buffer layer consists solely
of the materials of the matrices of said electroluminescent layers.

4. The organic light-emitting diode as claimed in one of the preceding
claims, in which said buffer layer exhibits a total thickness of between
1 and 10 nm.

5. The organic light-emitting diode as claimed in one of the preceding
claims, in which one of said electroluminescent layers is adapted for
emitting a blue light.

6. The organic light-emitting diode as claimed in claim 5, in which said
electroluminescent layers are chosen so as to allow the emission of white
light by the diode.

Description:

[0001] The invention pertains to an organic light-emitting diode
comprising at least two electroluminescent layers, and notably to such a
diode emitting a white light.

[0002] The color white is defined by the International Commission on
Illumination (CIE).

[0004] Whereas a "conventional" light-emitting diode consists of inorganic
semi-conducting materials, an OLED consists of a stack of layers of
organic materials, among which is included at least one
electroluminescent, fluorescent or phosphorescent layer. This results in
a fabrication technology which is much simpler and less expensive to
implement.

[0005] OLEDs emitting a white light can find application in domestic
lighting and in the back-lighting of flat screens (liquid-crystal or
filtered screens).

[0006] To generate a white light it is necessary to combine at least two
emitters of different colors. Consequently, white OLEDs comprise at least
two distinct electroluminescent layers, exhibiting two different emission
wavelengths, or else a single layer containing a mixture of two
electroluminescent materials also exhibiting two different emission
wavelengths.

[0007] FIG. 1 shows a flat band diagram of a white OLED with two spatially
separate fluorescent emitters. Depicted from left to right are an anode
A, a hole-transport layer HTL, a red electroluminescent layer ELR, a blue
electroluminescent layer ELB, an electron-transport layer ETL and a
cathode C. The rectangles represent the forbidden bands of the various
materials, lying between the HOMO ("highest occupied molecular orbital")
and LUMO ("lowest unoccupied molecular orbital") levels. The
electroluminescent layers are composed of a matrix and of a fluorescent
dopant; the dashed lines represent the HOMO and LUMO levels of the
dopants, the continuous lines those of the matrices. The reference NV
indicates the level in vacuo.

[0008] The chromatic coordinates of the two emitters must be situated, in
a CIE1931 diagram, on two points joined by a segment passing through the
region of said diagram corresponding to white (in the case of three or
more emitters: the polygon obtained by joining the points representing
the emission colors of the emitters must contain at least one portion of
the white region of the CIE1931 diagram).

[0009] However, the emission spectrum of a white light source that is to
be used as lighting is not characterized satisfactorily by just the
coordinates of the corresponding point in the CIE1931 chromatic space. It
is also necessary to take account of a parameter known as the color
rendering index (CRI). A satisfactory CRI requires that white be formed
on the basis of emitters whose representative points in the CIE1931
diagram are very remote. In particular, at least one of these emitters
must generate a light exhibiting a very saturated color. Generally, this
entails the blue emitter, thereby making it possible to minimize its
proportion of luminance while ensuring emission exhibiting the desired
chromatic coordinates. This choice makes it possible to limit the
electrical stress imposed on the blue organic emitters, which exhibit a
smaller lifetime than those emitting light of greater wavelength and thus
constitute the factor which limits the lifetime of white OLEDs. However,
it is also necessary to consider that, for one and the same luminance
level, the wider the forbidden band of the blue emitters, and therefore
the more saturated the blue color of the light they emit, the shorter
their lifetime. Consequently it is necessary to find a compromise, not
always satisfactory, between CRI and lifetime.

[0010] "Blue" emitters or emitters "with wide forbidden band" mean
electroluminescent materials exhibiting a forbidden band lying, by way of
indication, between 2.48 and 3.26 eV, corresponding to emission
wavelengths of between 380 and 500 nm. In a manner known per se, these
materials generally consist of a non-luminescent matrix (or host
material) containing a fluorescent or phosphorescent dopant. When one
speaks of the forbidden band of an emitter, one is referring to the
dopant; the forbidden band of the matrix is slightly wider.

[0011] The lifetime of an organic material with wide forbidden band is
limited by several physical and physicochemical phenomena.

[0012] Firstly, these materials oxidize easily on account of their LUMO
level being close to the void level.

[0013] Secondly, numerous blue emitters are stable in an ionic form and
deteriorate rapidly in the ionic form of opposite sign. Thus, these
materials need to be used to conduct electrons mainly (case of FIG. 1),
or holes mainly; the presence of minority carriers or of excitons (this
being necessary to produce light emission) causes a degradation of
electrochemical nature.

[0014] Thirdly, it is known empirically that these materials conduct
carriers relatively poorly, and therefore undergo significant electrical
stress.

[0015] These phenomena, which lead firstly to a progressive decrease in
luminance over time, and then to a failure of the diode, also occur when
blue emitters are used alone (in blue OLEDs), but in this case it is
possible to compensate for them, at least up to a certain point, through
a progressive increase in the supply voltage. On the other hand, in the
case of intimate association of two or more emitters, the difference in
their lifetimes causes a progressive and difficult-to-compensate
modification of the color point, that is to say of the chromatic
coordinates of the emitted light.

[0016] Furthermore, in the case of intimate association of two or more
emitters, the excitons generated in the emitter of large forbidden band
ELB have a tendency to diffuse toward the emitter of smaller forbidden
band ELR, reducing the luminance of the blue emitter ELB, which must
therefore be subjected to a higher electrical stress.

[0017] These problems are not solely specific to white

[0018] OLEDs, but affect all OLEDs comprising a blue emitter associated
with an emitter of narrower forbidden band.

[0019] It is known to separate the electroluminescent layers of a white
OLED (more generally, with two or more emitters) with an
electron-blocking and/or hole-blocking layer intended to control the
color point by modifying the distribution of the carriers in the device.
A blocking layer is defined by a less (more) energetic LUMO (HOMO) level
than the LUMO (HOMO) level of the adjacent layer on the cathode (anode)
side when dealing with an electron-blocking (hole-blocking) layer. A
barrier height or difference of electronic level is considered to have a
blocking effect provided that it is greater than 0.3 eV. This solution is
described in the article by CH. Kim and J. Shinar "Bright small molecular
white organic light-emitting devices with two emission zones" Appl. Phys.
Lett., Vol 80, no. 12 (2002) pp 2201-2203. It exhibits the drawback of
reducing the conductivity of the diode, since precisely the blocking
layer constitutes a barrier to the transport of the carriers.
Consequently, at equal luminance, a higher potential difference must be
applied across the terminals of the OLED, thereby limiting the gain in
terms of lifetime.

[0020] It is also known from the prior art to form the emitter layer as
two adjacent sub-layers of distinct host materials doped with the same
electroluminescent dopant, the whole being dubbed D-EML for Double
Emissive Layer (M. Ben Khalifa et al. "Efficient red phosphorescent
organic light emitting diodes with double emission layers", J. Phys. D:
Appl. Phys. 41 (2008) 155111). This organic junction exhibits the
advantage of associating a hole-conducting host on the anode side and an
electron-conducting host on the cathode side. Thus the carrier
recombination zone is situated at this junction, and limits the diffusion
of the excitons toward adjacent layers external to the D-EML. However, it
is very difficult, or even impossible, to apply this solution in the case
of a diode associating two or more emitters, since this makes it
necessary to considerably increase the number of evaporated layers (4 for
two emitters and 6 for three), and on the other hand since the sub-layers
in contact of the two or three D-EMLs must ensure non-negligible
conductivity both for the holes and for the electrons.

[0021] It is also known from the prior art to introduce an exciton
blocking layer provided that fluorescent and phosphorescent emitters are
associated in one and the same diode. See the article by Sung Hyun Kim,
Jyongsik Jang and Jun Yeob Lee "High efficiency phosphorescent organic
light-emitting diodes using carbazole-type triplet exciton blocking
layer" Applied Physics Letters 90, 223505, 2007. Indeed, phosphorescent
emitters are beneficial for low-luminance applications, where they
exhibit very high efficiencies. But as there is currently no
phosphorescent emitter with wide forbidden band exhibiting a sufficient
lifetime, it is not possible to produce an entirely phosphorescent white
light-emitting diode. Hence the benefit of associating a blue fluorescent
emitter and a phosphorescent emitter with greater wavelength. However, if
the precaution were not taken of separating the phosphorescent emitter
and the fluorescent emitter with an exciton blocking layer and these two
emitters were directly juxtaposed, the triplet excitons of the
phosphorescent material could diffuse toward the fluorescent material,
where they would de-excite in a non-radiative manner. An exciton blocking
layer typically exhibits a thickness of 3-10 nm, and a LUMO (HOMO) level
which is lower (higher) than that of the dopant of the phosphorescent
emitter. The exciton blocking layer, likewise, reduces the conductivity
of the OLED, necessitating the use of a high operating voltage which has
an unfavorable effect on the lifetime of the emitters.

[0022] The documents EP 1 784 056 and EP 1 936 714 disclose OLEDs
exhibiting a plurality of electroluminescent layers separated by
intermediate exciton blocking layers. The electroluminescent layers
consist of one and the same host material containing various dopants; the
intermediate layers consist of this same host material, but without
doping. As explained above, the exciton blocking layers reduce the
conductivity of the OLED, this having an unfavorable effect on the
lifetime of the emitters.

[0023] The document US 2009/0001875 discloses OLEDs comprising two
electroluminescent layers separated by an intermediate hole-conducting
layer. Once again, such a configuration can only reduce the conductivity
of the OLED, and consequently the lifetime of its emitters.

[0024] The documents DE 10 2007 058005 and US 2010/0314648 disclose OLEDs
exhibiting three electroluminescent layers separated by two buffer
layers, the latter consisting of a mixture of an electron-transporting
material and of a hole-transporting material.

[0025] The invention is aimed at overcoming the drawbacks of the prior art
so as to increase the lifetime of OLEDs comprising at least two
electroluminescent layers of one and the same type emitting at different
wavelengths.

[0026] In accordance with the invention, this aim is achieved by an
organic light-emitting diode comprising at least two electroluminescent
layers, both of fluorescent type or of phosphorescent type and emitting
at two different wavelengths, as well as an electron-and-hole conducting
buffer layer disposed between said electroluminescent layers,
characterized in that said buffer layer is a bilayer comprising an
electron-transport layer and a hole-transport layer, each of said
electron-transport and hole-transport layers consisting of one or more
materials whose HOMO level or levels lie between, or are equal to, the
HOMO levels of said electroluminescent layers, and whose LUMO levels lie
between, or are equal to, the LUMO levels of said electroluminescent
layers, this being with a tolerance of 0.3 eV.

[0027] The HOMO and LUMO levels are defined in a perfectly rigorous manner
for materials consisting of a single chemical species. In the case of
layers comprising a matrix and dopants, the HOMO and LUMO levels are
defined by extension as being those of the material constituting the
matrix. This definition is conventional.

[0028] The HOMO and LUMO levels of a material may be calculated by
numerical methods or measured by experimental techniques such as
photoelectric spectroscopy or cyclic voltametry. It is understood that
the energy levels of the various layers constituting an organic
light-emitting diode according to the invention must be defined with the
aid of one and the same method in order to be able to be inter-compared.
However, the choice of the method is immaterial, since only the
relationships between various energy levels are significant for the
implementation of the invention. In the examples hereinafter, it will be
considered that the HOMO and LUMO levels are defined by photoelectric
spectroscopy.

[0029] Preferably, said buffer layer can conduct, over at least a part of
its thickness, both electrons and holes.

[0030] Advantageously, said electroluminescent layers can comprise a
matrix and an electroluminescent dopant, and the two layers of said
bilayer can comprise a matrix of the same composition as that of the
closest electroluminescent layer.

[0031] Said electroluminescent layers can comprise a matrix and an
electroluminescent dopant, and said buffer layer can consist solely of
the material or materials of the matrices of said electroluminescent
layers, or of one of said electroluminescent layers.

[0032] Said buffer layer can exhibit a thickness of between 1 and 10 nm.

[0033] One of said electroluminescent layers may be adapted for emitting a
blue light. More particularly, said electroluminescent layers may be
chosen so as to allow the emission of white light by the diode.

[0034] Other characteristics, details and advantages of the invention will
emerge on reading the description offered with reference to the appended
drawings given by way of example and which represent, respectively:

[0035] FIG. 1, a flat band diagram of a white OLED according to the prior
art;

[0036] FIG. 2, a flat band diagram of a white OLED according to an
embodiment of the invention; and

[0037] FIG. 3, the structure of an OLED according to an embodiment of the
invention.

[0038] To increase the lifetime of a white OLED, or more generally an OLED
comprising at least two emitter layers, one of which is blue, the
invention proposes to insert, between said emitter layers, a buffer layer
allowing the transport of electrons and/or holes, and preferably of both
types of carriers, at least over a part of its thickness.

[0039] Preferably, the thickness of the buffer layer can be between 1 and
10 nm.

[0040] As illustrated in FIG. 2, this buffer layer T is in fact a bilayer,
formed by a first elementary layer T1, conducting holes, and a
second elementary layer T2, conducting electrons. In particular, the
first elementary layer can consist of the matrix of the red
electroluminescent layer ELR (more generally: said electroluminescent
layer having a narrow forbidden band), but without dopant; while the
second elementary layer can consist of the matrix of the blue
electroluminescent layer ELB, also without dopant. The converse is also
possible, at least in principle, but there are few blue emitters that
conduct holes.

[0041] Under these conditions, an exciton generated in one of the emitter
layers will diffuse with much greater difficulty toward the second
emitter on account of its remoteness. Furthermore, it is understood that
the main zone of recombination of the carriers is situated at the
interface between the two elementary layers, that is to say in an area
which is non-emissive and therefore has no real consequence on the
emission and if the materials of this zone deteriorate. This is a
considerable advantage with respect to the known embodiments of the prior
art, comprising a single buffer layer.

[0042] It is generally preferable that the buffer bilayer consist of the
same materials which form the matrices of the two emitter layers, but
this is not essential. It is therefore possible to produce a buffer
bilayer from materials other than those used in the electroluminescent
layers. In this case, it will be necessary to take care that this layer
does not constitute a carrier blocking layer. Accordingly, it is
necessary that the buffer layer consist of materials whose HOMO levels
are between, or equal to, the HOMO levels of said electroluminescent
layers, and whose LUMO levels are between, or equal to, the HOMO levels
of said electroluminescent layers.

[0043] The condition on the HOMO and LUMO levels shall not necessarily be
satisfied in an exact manner: a tolerance of about 0.3 V is permitted
since a barrier of this height is rendered negligible by the thermal
agitation of the carriers. Stated otherwise, HOMO or LUMO levels
exhibiting an energy difference of less than or equal to 0.3 eV are
considered to be "equal" within the meaning of the invention.

[0044] In all cases, the use of the buffer layer makes it possible to
limit both the diffusion of excitons and the formation of unstable ions
that are liable to rapidly degrade the OLED.

[0045] As the buffer layer is not an exciton blocking layer, the diode of
the invention need not associate a fluorescent emitter and a
phosphorescent emitter. The two emitters separated by the buffer layer
must exhibit an electroluminescence of the same type.

[0046] As shown by FIG. 4 (which is not to scale), an OLED according to
the invention typically takes the form of a stack of layers:

[0047] An
anode A, which may be made for example of AlCu/TiN, Cr, Mo, W, AlCu/W,
AlCu/Mo, Ag, ITO (indium-tin oxide) or of some other conducting
transparent oxide.

[0048] An optional hole injection layer HIL, which may
be made, for example, of CuPc, Pedot, or Pani.

[0049] A hole-transport
layer HTL, optionally doped to increase its conductivity and facilitate
the injection of carriers without needing to use an HIL, for example made
of SpiroTTB doped with F4TCNQ (1%), or NPB, TPD, Spiro TAD, etc. The
dopant can also be MoO3.

[0050] An optional electron blocking layer EBL,
which may be made, for example, of NPB, TPD, alpha NPD.

[0056] An optional electron injection layer EIL,
which may be made of LiF (lithium fluoride).

[0057] A cathode C of Al,
Mg/Ag, Ca, Ca/Ag, etc.

[0058] An optional layer CL making it possible to
improve the extraction of light through the cathode and/or the anode
("capping layer") in the case of a diode with emission through the upper
or transparent face. This layer may be, for example, of SiO, MoO3,
TeO2, ITO, SnO2, Sb2O3, ZnSe.

[0059] A concrete exemplary embodiment of the invention consists of the
following stack of layers:

[0060] A: AlCu (100 nm)/TiN: 10 nm

[0061]
HTL: SpiroTTB doped with F4TCNQ (1%): 32 nm

[0062] ELB: NPB doped with
Rubrene (1%): 5 nm

[0063] T1: NPB: 5 nm

[0064] T2: BH3: 5 nm

[0065] ELR: BH3 doped with BD3 (1.5%): 12 nm

[0066] HBL: Alq3: 2 nm

[0067] ETL: Bphen doped with Ca (4%): 42 nm

[0068] C: Ag: 15 nm

[0069]
CL: SiO: 25 nm.

[0070] An appreciable improvement in lifetime is noted in such a diode
with respect to conventional diodes exhibiting the same stack with the
exception of the buffer layers. This lifetime, defined as the time after
which the luminance is reduced by 50%, is improved by a factor of
typically between 5 and 10 for initial luminances of between 400 and 1500
Cd/m2.